Q9
Petroleum vapours are dangerous substances and when mixed with air can be ignited.
(a)
(i) sketch an explosimeter or combustion gas indicator which can be used to check the atmosphere of a tank or pumproom.
(5)
(ii) describe the explosimeter and its operation
(4)
(iii) state one reason that may cause the explosimeter to give a false reading
(1)
(b) for flammable mixtures, explain the meaning of the terms lower and upper flammable limits (
Reference Answer
### Part (a)(ii) Description and Operation of an Explosimeter An explosimeter, also known as a combustible gas indicator, is a portable instrument used to measure the concentration of flammable gases or vapours in the atmosphere. Its primary function is to determine if a space is safe for entry or hot work by indicating how close the atmosphere is to its Lower Flammable Limit (LFL), often expressed as a percentage (% LFL or % LEL). **1. Principle of Operation:** The most common type of explosimeter operates on the principle of catalytic combustion within a balanced Wheatstone bridge circuit. The heat generated by the combustion of a flammable gas on a catalytic filament causes a change in its electrical resistance, which unbalances the bridge. The degree of imbalance is proportional to the concentration of the flammable gas.
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